Optical system for recording and/or reproducing an optical information recording medium

ABSTRACT

An optical system for recording or reproducing information in an optical information recording medium, includes: a light source having a wavelength λ thereof; an objective lens; tracking device for tracking the objective lens by moving the objective lens in a direction perpendicular to an optical axis of the optical system; and a coupling lens optical system including at least one plastic lens and having a positive focal length, and being provided between the light source and the objective lens for guiding a light emitted from the light source to the objective lens. A maximum amount of change in a wavefront aberration of the coupling lens optical system in a numerical aperture corresponding to a maximum amount of tracking is 0.02 λrms or less.

BACKGROUND OF THE INVENTION

The present invention relates to an optical system in which light beamsfrom a light source are collected on an optical information recordingsurface to record and reproduce information on its surface, andspecifically, to an optical system in which influence by changes intemperature and humidity is suppressed.

Various optical systems have been proposed for a converging opticalsystem in which divergent light beams emitted from a light source areconverted into converging light beams and image-focused on an opticalinformation recording medium in order to record/reproduce information ona compact disk (CD), which is the most popular type of opticalinformation medium. A finite conjugation type optical system is common,in which a semiconductor laser having an oscillation wavelength ofapproximately 780 nm is used, and a single objective lens is used, inwhich the numerical aperture on the optical information recording mediumside is approximately 0.45, and both surfaces of which are aspherical,(refer to, for example, Japanese Patent Publication Open to PublicInspection No. 56314/1986). In many cases, the single objective lens iscomposed of resin, and its change due to temperature has not been aproblem because of the relatively small focusing distance and numericalaperture. Regarding also change due to humidity, even when resin havinga saturated water absorption ratio of approximately 1 to 2%, was used,there has been no problem.

Recently, however, increased density of information recording onto anoptical information recording medium such as optical disks, is advanced,and accompanying this, a reduction of wavelengths of the light source,and consequently a large increase of the NA of optical systems orobjective lenses are promoted. Specifically, for DVD, an extremeincrease of densities is advancing, and a wavelength of 635-650 nm and anumerical aperture of 0.6 are required for a converging light opticalsystem.

In order to record/reproduce such a high density optical informationrecording medium, a finite conjugation type optical system, composed ofa single objective lens, is not available because spherical aberrationis largely changed when the lens is moved for focusing due to the largenumerical aperture of the objective lens; and further, because muchastigmatism is generated when the lens is moved in the directionperpendicular to the optical axis for tracking.

In a system in which the light source and the objective lens areintegrally moved for focusing and tracking, there is no fluctuation ofaberration, however, the speed necessary for tracking or focusing cannot be obtained by a simple mechanism.

For these reasons, use of an optical system, composed of a light source,a coupling optical system and an objective lens, is necessary so thatthe objective lens is moved for focusing and tracking, and fluctuationof aberration caused by this movement is greatly suppressed, andthereby, an absolute value of a lateral magnification of the objectivelens can also be limited to less than a predetermined value.

Generally, as a coupling optical system, a collimator optical system isused to convert divergent light beams into parallel light beams. In thiscase, the lateral magnification mc of the coupling optical system iszero, when viewed from the opposite side of the light source.

As the collimator optical system, generally, a collimator lens of 1group including 2 elements is used, in which spherical glass surfacesare adhered to each other, and recently, a single aspherical glass lensis also used. Further, as a portion of the collimator optical system, asingle aspherical resin lens is also used.

Further, various types of collimator optical systems are also knownwhich are composed of single lenses having more than 3 elements, inwhich a positive lens made of low-dispersion material is combined with anegative lens made of high-dispersion material, in order to reduceinfluence due to fluctuations of the wavelength of semiconductor lasers.

Examples are disclosed in Japanese Patent Publication Open to PublicInspection Nos. 258573/1994 and 5909/1996, in which the divergencedegree of a divergent light beam, emitted from the light source, isreduced, and a lateral magnification of the objective lens is set to anegative value, which is close to zero, by using a coupling opticalsystem in which a lateral magnification mc, viewed from the oppositeside of the light source, is positive.

In the latter example, a double-sided aspherical resin lens isconsidered as the objective lens, and a single resin lens, at least onesurface of which is aspherical, is considered as the coupling opticalsystem.

In either example, counter measures to encounter a spherical aberration,which is a problem generated when the NA of the optical system andobjective lens is increased, and which is generated by changes of theenvironmental temperature and is varied by focusing, are described incases where the objective lens is made of resin.

In this connection, an example is disclosed in Japanese PatentApplication No. 352208/1995, in which a coupling lens having a negativelateral magnification mc, when viewed from the opposite side of thelight source, is used, and thereby, divergent light beams emitted fromthe light source are converted into converging light beams, and thelateral magnification of the objective lens is set to a value which ispositive and close to zero.

In this publication, conditions are described, which reduce thespherical aberration generated by changes of environmental temperature,to a degree which causes almost no problem even if the objective lens ismade of resin, with respect to a reduction of the wavelength of thelight source, and an increase of the NA of the optical system or theobjective lens, which are required for the optical system torecord/reproduce an extremely high dense optical information recordingmedium, such as a DVD. Further, the following is described: as thecoupling optical system, a single resin lens is used, and thereby, thespherical aberration generated by changes of the environmentaltemperature can further be reduced.

However, in the above examples, no countermeasure to overcome problemsgenerated by changes of ambient humidity is disclosed, when the lens ismade of resin, and therefore, it is not clear whether the optical systemcan allow humidity changes while recording/reproducing the highly denseoptical information recording media, such as DVDs.

Incidentally, an optical system is known which is composed of a glasslens and a plastic lens having a refractive power of approximately zero,as a collimator lens to convert the emitted light beams from asemiconductor laser into parallel light beams, which is used for anoptical system for an optical disk, so that a lens having excellent heatand humidity resistant performance can be provided (Japanese PatentPublication Open to Public Inspection Nos. 20377/1995 and 20378/1995).In the descriptions of the publications:

(1) There is a problem in which, when a collimator lens in an infiniteconjugation type optical system is composed of plastic, the focal lengthand the back focal length fluctuate greatly by changes of the refractiveindex caused by temperature change, and the position of the light sourceis out of the focus position for the lens, so that emitted light beamsare not parallel, and thereby, the optical performance is deteriorated.

(2) As an effect of the invention, a lens can be obtained in which thefocal length and the back focal length change only slightly when therefractive index of the plastic lens changes. Accordingly, fluctuationof the focal length is small with respect to temperature change, so thatthis lens can be used when heat resistant performance is required.Further, regarding also a change of humidity, the refractive index ofthe lens is changed base on the humidity absorption, however, the changeof focal length of the lens is small with respect to this refractiveindex, and this lens also has humidity resistant characteristics.

In the above item (1), when only a single plastic collimator lens isconsidered, it is a fact that the focal length and the back focal lengthare changed with a change of the refractive index due to temperaturechange. However, in the whole optical system for recording/reproducingan optical information recording medium, which is composed of a lightsource, a single plastic collimator lens, and an infinite conjugationtype objective lens, even if the emitted light beams from the collimatorlens are not parallel caused by a change of the refractive index due totemperature change, the fluctuation of the spherical aberration issmall.

Incidentally, in the above optical system, the objective resin lens haspositive spherical aberration because the refractive index of resinmaterials decreases when the ambient temperature rises. On the otherhand, in the single collimator resin lens, the spherical aberrationgenerated in the single collimator lens is small when the refractiveindex of the resin material decreases due to temperature rise, and as aninfluence of the temperature rise, emitted light beams from the singlecollimator lens become divergent light beams because the back focallength is greater. Incidentally, in the objective lens, negativespherical aberration is generated when the divergent light beam entersinto the lens. Accordingly, the collimator lens is composed of a singleresin lens, and thereby, the change of the spherical aberration due to achange of the ambient temperature in the objective resin lens, iscorrected.

Further, in the item (2) above, it is a fact that a change of the focallength and back focal length is decreased by causing the refractiveindex of the plastic lens to be approximately zero when the whole lensis uniformly under dehumidified or humidified condition. However, in amoisture absorption process following the uniform dehumidifyingcondition, or in a dehumidifying process following the uniform moistureabsorption condition, unevenness of a refractive index distribution isgenerated in the lens, and the spherical aberration largely changes in alens of the rotational symmetry around the optical axis. This fact iswidely known for single objective resin lenses. (Refer to P. 74 ofKONICA TECHNICAL REPORT 3, A moisture absorption simulation of a plasticobjective lens for an optical disk.)

Regarding also a single collimator resin lens, or a resin lens,constituting a coupling optical system, regardless of its refractivepower, its lateral magnification or its numerical aperture, theinfluence of the refractive index distribution in the lens is presumedin the same manner as in the single objective resin lens.

In fact, in cases of a laser disk, which has a lower recording densitycompared to a DVD, a light source having a wavelength of approximately780 nm, and an indefinite conjugation type objective lens having a focallength of 4.5 mm, and having a numerical aperture of 0.50 (an effectiveaperture is 4.5 mm) are used, and as a collimator lens, a lens having afocal length of 17.0 mm, and a numerical aperture on the lightsource-side of 0.14, is used.

In both an objective lens and a collimator lens, the diffraction limitedperformance is obtained by a combination spherical glass lens,respectively composed of a 2-group containing 3-element lens, and a1-group containing 2-element lens.

In the objective lens, the numeral aperture is large, and thecombination spherical glass lens is expensive. Accordingly, a plasticaspherical lens is adopted first of all. Following this lens, as a partof the collimator lens, a plastic aspherical lens of the followingspecification is adopted.

Wavelength to be used: 780 nm

Focal length: 17.0 mm

Effective aperture: 4.76 mm (5.7 mm)

Numerical aperture on the light source side: 0.14 (0.17)

Lens outer diameter: 7.75

Lens material: acrylic resin (saturated water absorption rate α=1.0%)

Standard of spherical aberration: within 0.031 λrms (at NA 0.14)

(Numerical values in parentheses of the above effective aperture and thenumerical aperture on the light source side respectively correspond tothe case where this lens is combined with the above-described lens, andthe amount of tracking is 0.6 mm)

A sectional view of this collimator lens 30 is shown in FIG. 2.

The result of a high humidity test of this collimator lens is shown inFIG. 3.

The test condition was as follows: after the lens had been placed in aconstant temperature and constant humidity tank at a temperature of +60°C., and relative humidity of 90% for 120 hours, the condition wasrestored to an environmental condition of normal temperature and normalhumidity, and changes of wave front aberration were measured with aninterferometer for 384 hours. The light source of the interferometer wasa He-Ne laser (wavelength is 633 nm).

As can be seen from FIG. 3, the spherical aberration component changedby approximately 0.025 λrms during dehumidification. When this value isconverted into the actual 780 nm wavelength of the light source, thechanged amount is 0.02 λrms. When the residual spherical aberration ofthe collimator lens is approximately equal to the limits of thestandard, the maximum amount of the spherical aberration reaches 0.051λrms, however, the collimator lens can be used for a laser disk.However, in the optical system for recording/reproducing a high densityoptical information recording medium such as a DVD, such sphericalaberration and its accompanying fluctuations due to humidity absorptionare feared, and therefore, a combination spherical glass lens or asingle aspherical glass lens, which are expensive, is being used as acoupling optical system.

SUMMARY OF THE INVENTION

In the present invention, an optical system for recording/reproducing anextremely high density optical information recording medium, such as aDVD, comprises a light source, a coupling optical system and anobjective lens; and tracking is carried out by moving the objective lensin the direction perpendicular to the optical axis. In this case, evenwhen a part or the entire coupling optical system is composed of plasticlenses, an optical system can be obtained in which the influence ofchange in humidity can be suppressed to an allowable degree.

In the present invention, an optical system for recording and/orreproducing an optical information recording medium is characterized inthat: the optical system is provided with a coupling optical systemwhich is located between a light source and an objective lens, and whichhas a positive focal length so as to guide a light beam from the lightsource to the objective lens; and the optical system carries outtracking by moving the objective lens in the direction perpendicular tothe optical axis of the coupling optical system, wherein the couplingoptical system comprises at least one plastic lens, and the maximumamount of change of the wave front aberration due to water absorption ofthe plastic lens in the coupling optical system is not larger than 0.02λrms in a numerical aperture corresponding to the maximum tracking.

Accordingly, the plastic lens in the coupling optical system ischaracterized in that it is composed of materials having a saturatedwater absorption ratio of not more than 0.5%, concretely made ofpolyolefine resin or norbornene resin.

In this optical system, the amount of maximum tracking is between 0.1 mmand 0.7 mm, the numerical aperture NA₀ on the recording medium side ofthe objective lens is NA₀ >0.52, and the wavelength λ of the lightsource is not more than 700 nm.

The coupling optical system is preferably composed of a single plasticcoupling lens, and the single plastic coupling lens is characterized inthat the amount of the maximum change of the wave front aberration ofthe single coupling lens by water absorption is not more than 0.02 λrmsin the numerical aperture required at maximum tracking of the objectivelens. The single plastic coupling lens is further characterized in thatit is made of materials having a saturated water absorption ratio of notmore than 0.5%, concretely, made of polyolefine resin or norborneneresin, and the focal length f_(c) is 12 mm<f_(c) <36 mm.

FIG. 4(a) shows a model of aberration in the case where the couplingoptical system has the spherical aberration. In the drawing, theabscissa axis W shows the wave front aberration, and the ordinate axis ρshows the distance from the optical axis which is normalized by theeffective aperture of the objective lens. When the objective lens isplaced on the optical axis, the wave front in a range B is used, whichis symmetrical about the optical axis, in the wave front emitted fromthe coupling optical system. However, when the objective lens is moved Δby tracking in the direction perpendicular to the optical axis, the wavefront in the range C shifted by Δ, as shown in FIG. 4(b), is used.

The wave front in the range C is asymmetrical about the optical axis ofthe objective lens, and actually, coma is generated.

The wave front W in FIG. 4(a) is expressed as follows:

    W=A ρ.sup.4                                            ( 1)

where A is a coefficient.

When the objective lens is moved Δ by tracking perpendicular to theoptical axis, the wave front W is expressed by the following equation.##EQU1## The first term expresses the spherical aberration, and thesecond term expresses the coma. That is, the equation shows that thecoma, which greatly influences the recording/reproducing performance, isgenerated by tracking.

The spherical aberration component W_(sa) and the coma component W_(cm)in the rms value of the wave front aberration are expressed as follows:##EQU2##

It is assumed that an objective lens for DVD of the followingspecification is used in combination with the conventional collimatorlens.

Wavelength to be used 635 nm

Focal length 3.36 mm

Effective aperture 4.03 mm

Numerical aperture on the disk-side 0.6

Outer diameter of the lens 5.8 mm

NA on the light source side required for a collimator lens having afocal length of 17.0 mm, is 0.118 when the amount of tracking of theobjective lens is 0, and 0.148 when the maximum amount of tracking is0.5 mm. When ρ=1 at NA=0.118, and Δ, corresponding to the maximum amountof tracking, is Δ_(max), then

    Δ.sub.max =(0.148-0.118)/0.118=0.25.

The maximum amount of tracking of DVD is assumed to be 0.1-0.7 mm. Atpresent, the eccentricity of the disk is severely controlled, andtherefore, the maximum amount of tracking of DVD is acceptable at about0.2 mm. However, it is presumed that, in the future, the request forrecording/reproducing disks of out of the standard is increased, and inthis case, the maximum amount of tracking is required to be as large aspossible. However, when it is larger than 0.7 mm, the overall sizebecomes larger, and further, it is difficult to obtain acceptablemechanical characteristics of the tracking mechanism.

When NA is 0.14, the amount of change of the spherical aberrationcomponent of the wave front aberration of the collimator lens atdehumidifying is 0.025 λrms, and since the spherical aberrationcomponent is proportional to the 4th power of NA, the amount of changeof the spherical aberration component is 0.0127 λrms at a NA of 0.118.

Accordingly, when A is found from the expression (3), ##EQU3##

From the expression (4), the coma component generated by tracking is##EQU4##

On the other hand, the standard of the spherical aberration remaining inthe collimator lens is not more than 0.0127 λrms under the condition ofthe wavelength of 635 nm and NA of 0.118.

Accordingly, when tracking is 0.5 mm, a coma of 0.02 λrms is furthergenerated. Due to this, even if there is no coma in the collimator lensand the objective lens, a coma of 0.04 λrms is generated by theremaining spherical aberration and the fluctuation of the sphericalaberration of the collimator lens by the moisture absorption anddehumidification. In the optical disk, this coma is generated in thedirection of adjoining track, however, in the DVD, because the pitch ofthe track is highly dense and narrower than that of the laser disk, thecollimator lens in the present example shown in FIG. 2 can not be used.

Generally, in the objective lens, because the numerical aperture islarge, the coma is generated by the eccentricity of the lens, notdepending on the single aspherical lens or the combination lens, and thestandard for the coma is generally not larger than 0.03 λrms. Further,sometimes the coma generated in the objective lens is corrected bytilting the objective lens. However, the coma generated by tracking theobjective lens can not be corrected because the coma is generatedcorresponding to the amount of tracking.

In order to suppress the coma, generated corresponding to the maximumtracking amount of less than 0.03 λrms, if the standard for thespherical aberration remaining in the coupling optical system is thesame, the coma component W_(cm), a, which is generated when thespherical aberration is fluctuated by W_(sa), a by moisture absorptionand dehumidification, and when tracking is carried out, is required tobe less than 0.01 λrms.

From equations (3) and (4), ##EQU5## then, W_(sa), a =0.0063 λrms.

This value means that the change of the spherical aberration component,at a numerical aperture of 0.148 corresponding to the maximum trackingamount of 0.5 mm, is 0.016 λrms.

That is, the amount of change of the transient wave front aberrationcaused by moisture absorption and dehumidification, may be approximatelyless than 0.02 λrms when considering aberrations other than thespherical aberration, at the numerical aperture corresponding to themaximum tracking amount (for example, 0.5 mm) in the coupling opticalsystem of the optical system for recording/reproducing a high densityoptical information recording medium, such as a DVD.

A measuring method for the maximum changing amount of the wavefrontaberration of the plastic lenses or the lens system including plasticlenses at the numerical aperture corresponding to the time of maximumtracking, will be described below.

After the measuring object has been stored for 168 hours in the constanttemperature and humidity tank, under the +60° C. temperature and the 90%relative humidity, the wavefront aberration of the object is measured byan interferometer for 384 hours, after return to the normal temperatureof 25° C. and the normal humidity of 50% relative humidity, and thedifference between the maximum wavefront aberration and the minimumwavefront aberration is defined as the maximum changing amount of thewavefront aberration.

Herein, the numerical aperture corresponding to the time of maximumtracking means the numerical aperture on the side of light source of theplastic lenses or the lens system including plastic lenses, that is, thenumerical aperture corresponding to the maximum luminous flux which canbe changed by tracking of the objective lens.

FIG. 5 is an illustration of the overall optical system. Herein, thenumerical aperture of a coupling lens (13) corresponding to the time ofthe maximum tracking, designates "A" in the drawing. In this connection,"B" in the drawing represents the numerical aperture when the trackingamount is "0", and a light beam at the extreme periphery (EP) of theluminous flux corresponding to the numerical aperture, is regulated byan aperture-stop (5) so as to correspond to the numerical aperture NA₀on the side of an information recording medium (7) of an objective lens(16). Herein, a light beam at the extreme periphery of the luminous fluxat the time of "A" in the drawing, is equivalent to the case in which alight beam at the extreme periphery of the luminous flux at the time of"B" in the drawing, is vertically shifted by Δmax by tracking, in thedrawing.

Measurement of the wavefront aberration is carried out using, forexample, Twyman-Green interferometer as shown in FIG. 6. In themeasurement of the wavefront aberration, the same wavelength as that ofthe light source, used for the recording/reproducing optical system ofthe actual optical information recording medium, is used.

However, when the wavelength of the light source of the interferometeris not equal to that of the light source, used for therecording/reproducing optical system of the actual optical informationrecording medium, the wavefront aberration can be measured by thefollowing methods: a correction plate CP is inserted between a referenceconcave mirror CM of the interferometer and the measuring object 13, orthe lateral magnification of the measuring object is shifted, so thatthe same effects as the correction of wavelength is obtained. Further,the result of measurement can also be converted by the optical designsimulation.

In order to realize such a recording/reproducing optical system for theoptical information recording medium by improving a characteristic of aresin material, a saturated water absorption ratio α may be less than0.5%. In this connection, it is preferable that α=0% in order to fullysatisfy the standard of the remaining spherical aberration component forthe coupling lens. As a test method of the saturated water absorptionratio, ASTM (American Society for Testing Materials) D570 (the testcondition: a sample is placed in 23° C. water for a week) was employed.

There are various types of resin materials for an optical use with an αof less than 0.5%. As a resin with a relatively small birefringence,polyolefine resin such as Zeonex (trade name) by Nihon Zeon Co. or APEL(trade name) by Mitsui Petrochemical Industries, or a norbornene resinrepresented by ARTON (trade name) by Japan Synthetic Rubber Co., ispreferable.

As an objective lens for DVD use, the following characteristics arerequired: a working distance of more than 1.0 mm is kept on the diskhaving the thickness of 0.6 mm and the overall size is as small aspossible. Further, for CDs, the minimum moving distance of 1.6 mm isnecessary for a 1.2 mm thick disk when the same objective lens is movedalong the optical axis for reproduction.

When considering the above case, and in a case where the infiniteconjugate type aspherical single objective lens is used for theobjective lens, the focal length between 1.8 and 5 mm is required, andit is preferable to set the focal length between 2.4 and 4.5 mm.

The focal length of 1.8 mm corresponds to the case where the necessaryminimum moving distance is kept for the exclusive use of DVD, and thefocal length of 5 mm corresponds to the case where DVDs and CDs can beinterchanged with each other, and the plastic lens is produced by a sidegate molding system. When the focal length is longer, the overall sizeof the optical system becomes larger. Further, when the focal length isshorter, molding using special glass material with a large refractiveindex becomes necessary.

The lateral magnification m_(t) of the entire optical system is between-1/10 and -1/4, and it is more preferable that it be set between -1/8and -1/5.

From the above requirement, in the case where the objective lens is aninfinite conjugation type, the focal length f_(c) of the couplingoptical system is

    7.2 mm<f.sub.c <50 mm,

and more preferably,

    12 mm<f.sub.c <36 mm.

Incidentally, normally, the objective lens is moved along the opticalaxis for focusing. In this case, when the lateral magnification |m₀ | ofthe objective lens is large, fluctuation of the spherical aberration ofthe objective lens by focusing becomes a problem, and therefore,

    |m.sub.0 |<1/10 is preferable.

Accordingly, when m₀ is not 0, the lateral magnification m_(c), viewedfrom the objective lens side of the coupling optical system, is obtainedfrom the following relationship:

    m.sub.c =m.sub.0 /m.sub.t.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an optical arrangement of Example 1 of anoptical system for recording/reproducing an optical informationrecording medium of the present invention.

FIG. 2 is a sectional view showing an example of a conventionalcollimator lens.

FIG. 3 is a graph showing results of a high humidity test of thecollimator lens single body in FIG. 2.

FIG. 4(a) and 4(b) are illustrations showing the influence of trackingin the case where the coupling optical system has spherical aberration.

FIG. 5 is a schematic view showing the entire system of an example ofthe optical system for recording/reproducing the optical informationrecording medium of the present invention.

FIG. 6 is a view of an optical path showing a composition of aTwyman-Green interferometer used to measure the wave front aberration.

FIG. 7 is a graph showing the change of the wave front aberration bytracking in the optical system of Example 1 of the present invention.

FIG. 8 is a graph showing the change of the wave front aberration withrespect to the amount of tracking when the temperature of the opticalsystem rises 30° C. in the optical system of Example 1 of the presentinvention.

FIG. 9 is a graph showing the change of the wave front aberration by thechange of environment in a collimator lens of the optical system ofExample 1 of the present invention.

FIG. 10 shows a view of the optical arrangement of Example 2 of theoptical system for recording/reproducing the optical informationrecording medium of the present invention.

FIG. 11 is a graph showing the change of the wave front aberration bytracking in the optical system of Example 2 of the present invention.

FIG. 12 is a graph showing the change of the wave front aberration withrespect to the amount of tracking when the temperature of the opticalsystem rises 30° C. in the optical system of Example 2 of the presentinvention.

FIG. 13 is a graph showing the change of the wave front aberration dueto environmental changes in the coupling lens of the optical system ofExample 2 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Examples of the present invention will be described below. Thewavelength used in those examples is 635 nm. The symbols in the tablesrepresent the following:

ri: radius of curvature at the vertex of the i-th lens surface from thelight source

di: surface interval of the i-th lens from the light source

ni: refractive index of lens material of the i-th lens from the lightsource

f_(c) : focal length of the collimator lens

f₀ : focal length of the objective lens

U: distance between the object and the image of the optical system

T: distance to the light source when viewed from the first surface ofthe optical system.

The shape of the aspherical surface is expressed by a following equationwhen the vertex of the surface is an origin, the curvature of the vertexis C, a coefficient of the cone is κ, a coefficient of the asphericalsurface is Ai, and an exponent of the aspherical surface is Pi (≧4).##EQU6##

Example 1

As the coupling optical system, a collimator lens 3 by which a luminousflux from the light source 1 is converted into parallel light beams, isadopted, and is combined with an infinite conjugation type objectivelens 6.

When f_(c) =25.2 mm and f₀ =3.37 mm, then m_(t) =-1/7.5, T=-22.557 mm,and U=35.973 mm.

    ______________________________________                                        Surface No.   ri          di     ni                                           ______________________________________                                        1 cover glass ∞     0.95   1.51455                                      2 cover glass ∞     1.00                                                3 collimator lens                                                                           161.627     1.70   1.53830                                      4 collimator lens                                                                           -14.753     5.00                                                5 aperture-stop                                                                             ∞     0.00                                                6 objective lens                                                                            2.155       2.60   1.53830                                      7 objective lens                                                                            -6.594      1.566                                               8 transparent substrate                                                                     ∞     0.60   1.58000                                      9 transparent substrate                                                                     ∞                                                         ______________________________________                                        Aspherical coefficient                                                        ______________________________________                                        4th surface                                                                   κ = -7.09000 × 10.sup.-1                                          6th surface                                                                   κ = -9.90670 × 10.sup.-1                                          A.sub.1 = 6.06760 × 10.sup.-3                                                            P.sub.1 = 4.0000                                             A.sub.2 = 2.43360 × 10.sup.-4                                                            P.sub.2 = 6.0000                                             A.sub.3 = 6.88550 × 10.sup.-6                                                            P.sub.3 = 8.0000                                             A.sub.4 = -5.62880 × 10.sup.-6                                                           P.sub.4 = 10.0000                                            7th surface                                                                   κ = -2.73090 × 10.sup.-1                                          A.sub.1 = 9.23170 × 10.sup.-3                                                            P.sub.1 = 4.0000                                             A.sub.2 = -4.00540 × 10.sup.-3                                                           P.sub.2 = 6.0000                                             A.sub.3 = 8.31010 × 10.sup.-4                                                            P.sub.3 = 8.0000                                             A.sub.4 = -7.33530 × 10.sup.-5                                                           P.sub.4 =10.0000                                             ______________________________________                                    

A sectional view of Example 1 is shown in FIG. 1. In FIG. 1, a lightbeam emitted from a light source 1 passes through a cover glass 2, thenthrough a collimator lens 3, to turn into the light beam which is almostcollimated, and then it is limited to a prescribed light beam by anaperture-stop 5 and enters an objective lens 6. The light beam enteredthe objective lens 6 is converged on an information recording surface 8through a transparent substrate 7. In this optical system, results of asimulation of a change of the wave front aberration are respectivelyshown in FIG. 7, with respect to the amount of tracking in a case wherethe collimator lens 3 is made as a design value (non-aberration), andthat in the case where the collimator lens itself has a three-orderspherical aberration of 0.02 λrms, with respect to an effective apertureof 5.04 mm of the collimator lens, considering an amount of tracking of0.5 mm.

When the collimator lens 3 has no-aberration, the wave front aberrationdoes not change by tracking. On the other hand, when the collimator lens3 has a three-order spherical aberration, the wave front aberration isdeteriorated by tracking.

FIG. 8 shows the result of a simulation of a change of the wave frontaberration with respect to an amount of tracking when temperature of theoptical system is raised by 30° C. (ΔT=30° C.). In this case, since thecollimator lens 3 and the objective lens 6 are made of resin, it isassumed that the refractive indices respectively change by -0.0036.Although the spherical aberration of the entire optical system changes,the change of the wave front aberration by tracking is small.

The collimator lens 3 is made of a material having a saturated waterabsorption ratio α of less than 0.1%. FIG. 9 shows results in which acollimator lens 3 was stored in a constant temperature and humidity tankof +60° C. temperature and 90% RH relative humidity for 168 hours; thenits conditions are restored to the normal temperature and humidityenvironmental condition; and changes of the wave front aberration aremeasured by an interferometer. Since, as the light source 1, a He-Nelaser with 633 nm wavelength is used, its wavelength is approximatelyidentical to the design work wavelength, obviating conversion. Forconvenience of the measurement, an aperture-stop matched with theluminous flux at the extreme periphery on the optical axis is used.Under these conditions, the entire wave front aberration andfluctuations of spherical aberration components in the wave frontaberration are less than 0.005 λrms, and even if all of fluctuations areassumed to be fluctuations of the spherical aberration, they are lessthan 0.015 λrms.

As described above and as can clearly be seen in FIGS. 7, 8 and 9,deterioration of the tracking characteristic due to humidity changes issmall even in a high density medium such as a DVD, and an optical systemhaving also the excellent temperature characteristic can be realized atlower cost.

Example 2

As the coupling optical system shown in FIG. 10, a coupling lens 13 isadopted, by which luminous flux from the light source 1 is convertedinto convergent light beams, and is combined with an objective lens 16which is conjugate for convergent light beams.

    f.sub.c =18.1 mm, f.sub.0 =3.80 mm

A imaging magnification ratio m_(c), viewed from the objective lens sideof the coupling lens 13, and a magnification ratio m₀ of the objectivelens 16 are respectively given as follows:

m_(c) =-0.63 and m₀ =1/12, then m_(t) =-1/7.5, T=-26.84 mm, and U=40.18mm.

    ______________________________________                                        Surface No.   ri          di     ni                                           ______________________________________                                        1 cover glass ∞     0.95   1.51455                                      2 cover glass ∞     1.30                                                3 coupling lens                                                                             23.567      1.70   1.53830                                      4 coupling lens                                                                             -16.170     4.62                                                5 aperture-stop                                                                             ∞     0.00                                                6 objective lens                                                                            2.260       2.60   1.53830                                      7 objective lens                                                                            -12.703     1.57                                                8 transparent substrate                                                                     ∞     0.60   1.5000                                       9 transparent substrate                                                                     ∞                                                         ______________________________________                                        Aspherical coefficient                                                        ______________________________________                                        3rd surface                                                                   κ = -4.13770                                                            4th surface                                                                   κ = -6.11760 × 10.sup.-1                                          A.sub.1 = 2.60960 × 10.sup.-5                                                            P.sub.1 = 4.0000                                             6th surface                                                                   κ = -9.01750 × 10.sup.-1                                          A.sub.1 = 5.40980 × 10.sup.-3                                                            P.sub.1 = 4.0000                                             A.sub.2 = 2.97160 × 10.sup.-4                                                            P.sub.2 = 6.0000                                             A.sub.3 = 1.63600 × 10.sup.-5                                                            P.sub.3 = 8.0000                                             A.sub.4 = -2.71680 × 10.sup.-6                                                           P.sub.4 = 10.0000                                            7th surface                                                                   κ = -2.25470 × 10                                                 A.sub.1 = 1.19200 × 10.sup.-2                                                            P.sub.1 = 4.0000                                             A.sub.2 = -4.39840 × 10.sup.-3                                                           P.sub.2 = 6.0000                                             A.sub.3 = 8.74010 × 10.sup.-4                                                            P.sub.3 = 8.0000                                             A.sub.4 = -7.46390 × 10.sup.-5                                                           P.sub.4 = 10.0000                                            ______________________________________                                    

A sectional view of Example 2 is shown in FIG. 10. In FIG. 10, a lightbeam emitted from a light source 1 passes through a cover glass 2, thenthrough a coupling lens 13, to turn into the light beam which isconverged, and then it is limited to a prescribed light beam by anaperture-stop 5 and enters an objective lens 16. The light beam enteredthe objective lens 16 is further converged on an information recordingsurface 8 through a transparent substrate 7. In this optical system, theresults of a simulation of a change of the wave front aberration arerespectively shown in FIG. 11 with respect to the amount of tracking ina case where the coupling lens 13 is made as a design value(non-aberration), and that in the case where the coupling lens itselfhas a three-order spherical aberration of 0.02 λrms, with respect to aneffective aperture of the coupling lens 13, considering an amount oftracking of 0.5 mm.

The wave front aberration changes by tracking also when the couplinglens 13 has no-aberration. This aberration mainly includes off-axialaberration, apparently, which is generated when the light source isshifted from the optical axis of the objective lens 16, and when theoffense against sine condition of the objective lens 16 is corrected bya well-known method, astigmatism is a main component. On the other hand,when the coupling lens 13 has a third order spherical aberration, thecoma is deteriorated by tracking, in addition to the astigmatism.

FIG. 12 shows the result of simulation of a change of the wave frontaberration with respect to an amount of tracking when temperature of theoptical system rises 30° C. (ΔT=30° C.). In this case, because both ofthe coupling lens 13 and the objective lens 16 are made of resin, it isassumed that their refractive indices respectively change by -0.0036.The spherical aberration of the entire optical system changes, however,the change of the wave front aberration by tracking is small.

The coupling lens 13 is made of a material having a saturated waterabsorption ratio α of less than 0.1%. FIG. 13 shows a result in which acoupling lens 13 was stored in a constant temperature and humidity tankof +60° C. temperature and 90% RH relative humidity for 168 hours; thenthe lens conditions were restored to normal temperature and humidityenvironmental condition; and changes of the wave front aberration weremeasured with an interferometer. Since, as the light source 1, a He-Nelaser with a 633 nm wavelength is used, its wavelength is approximatelyidentical to the design wavelength, so that conversion is obviated. Forconvenience of measurement, an aperture-stop 5 matched with the luminousflux at the extreme periphery on the optical axis is used. Under theseconditions, the entire wave front aberration and fluctuations ofspherical aberration component in the wave front aberration are lessthan 0.005 λrms, and even if all fluctuations are assumed to befluctuations of the spherical aberration, they are less than 0.015 λrms.

As described above, and as can clearly be seen in FIGS. 11, 12 and 13,deterioration of the tracking characteristic due to humidity changes issmall even in a high density medium such as a DVD, and an opticalsystem, having also an excellent temperature characteristic, can berealized at lower cost.

Although in the above examples, cases in which the lateral magnificationof the objective lens m₀ =0 and m₀ >0, are shown, a case in which m₀ <0,is the same as above.

That is, a coupling lens may be adopted, by which light beam from thelight source is converted into divergent light beam, and is combinedwith an objective lens which is conjugate for the divergent light beam.

Further, a case in which the coupling optical system is composed of asingle lens, is described above. However, when the coupling opticalsystem is composed of a plurality of single lenses, and some of thelenses are made of resin so as to correct the chromatic aberration, theresin lens may satisfy the above conditions.

Further, in order to attain less than 0.02 λrms in the numericalaperture in which the maximum changing amount of the wave frontaberration due to water absorption of the plastic lens in the couplingoptical system corresponds to maximum tracking, a method to eliminatethe influence of the refractive index distribution in the waterabsorption/dehumidification process by considering the outer shape,thickness, or by coating a material to relax the water absorption on theouter shape or lens surface, other than utilization of resin materialwith low saturated water absorption ratio, or similar methods, can alsobe used.

In the recording/reproducing optical system of a type in which the CD isalso used as the DVD, by moving the coupling optical system along theoptical axis, the present invention can be attained by considering themaximum changing amount of the wavefront aberration when the numericalaperture on the side of the light source of the coupling optical systemis large.

As can clearly be seen from examples and characteristic views, accordingto the present invention, when a plastic lens in the coupling opticalsystem is made of a material in which deterioration of the wave frontaberration by water absorption is small, even when the coupling lens andthe objective lens are respectively composed of single lenses,deterioration of the wave front aberration by tracking can besuppressed, and thereby, a desirable optical system for reproducing ahigh density information recording medium such as a DVD, can beobtained.

What is claimed is:
 1. An optical system for recording or reproducinginformation in an optical information recording medium, comprising:(a) alight source having a wavelength λ thereof; (b) an objective lens; (c)tracking means for tracking the objective lens by moving the objectivelens in a direction perpendicular to an optical axis of the opticalsystem; and (d) a coupling lens optical system including at least oneplastic lens and having a positive focal length, and being providedbetween the light source and the objective lens for guiding a lightemitted from the light source to the objective lens; wherein a maximummount of change in a wavefront aberration of the coupling lens opticalsystem in a numerical aperture corresponding to a maximum amount oftracking is 0.02 λrms or less.
 2. The optical system of claim 1,whereina saturation water absorbing rate of the plastic lens included in thecoupling lens optical system is 0.5% or less.
 3. The optical system ofclaim 2,wherein the plastic lens included in the coupling lens opticalsystem is made of either polyolefine resin or norbornene resin.
 4. Theoptical system of claim 1, wherein the maximum amount of tracking of thetracking means is 0.1 mm to 0.7 mm.
 5. The optical system of claim 1,wherein a numerical aperture NA₀ of the objective lens on a side of theoptical information recording medium satisfies the following expression,

    NA.sub.0 >0.52.


6. The optical system of claim 1, wherein the wavelength λ of the lightsource is 700 nm or less.
 7. A coupling lens optical system for use inan optical system for recording or reproducing information in an opticalinformation recording medium, the coupling lens optical system having apositive focal length and being provided between a light source having awavelength λ and an objective lens which moves for tracking in adirection perpendicular to an optical axis of the optical system forrecording or reproducing, for guiding a light emitted from the lightsource to the objective lens, the coupling lens optical systemcomprising:a plastic lens, wherein a maximum amount of change in awavefront aberration of the coupling lens optical system in a numericalaperture corresponding to a maximum amount of tracking is 0.02 λrms orless.
 8. The coupling lens optical system of claim 7,wherein asaturation water absorbing rate of the plastic lens included in thecoupling lens optical system is 0.5% or less.
 9. The coupling lensoptical system of claim 8,wherein the plastic lens included in thecoupling lens optical system is made of either polyolefine resin ornorbornene resin.
 10. The coupling lens optical system of claim7,wherein a numerical aperture NA₀ of the objective lens on a side ofthe optical information recording medium satisfies the followingexpression,

    NA.sub.0 >0.52.


11. The coupling lens optical system of claim 7,wherein the wavelength λof the light source is 700 nm or less.
 12. The coupling lens opticalsystem of claim 7,wherein the coupling lens optical system is a plasticsingle lens.
 13. The coupling lens optical system of claim 12,wherein asaturation water absorbing rate of the plastic single lens is 0.5% orless.
 14. The coupling lens optical system of claim 12,wherein theplastic single lens is made of either polyolefine resin or norborneneresin.
 15. The coupling single lens of claim 12, wherein a focal lengthf_(c) of the plastic single lens satisfies the following expression,

    12 mm<f.sub.c <36 mm.